Electro-optical display devices are the object of considerable research efforts. Of the various display systems that have been developed, thin flat-panel display devices utilizing, for example, liquid crystal components are of particular commercial interest.
Compositions characterized as liquid crystals include a wide range of materials. The different electrical and optical properties exhibited by these liquid crystalline materials make possible a number of mechanisms for light modulation. Such mechanisms include phase transitions, dynamic scattering, and field effects, all of which are well known in the art.
Field effect devices are of particular utility. The effect that is commercially most significant at present is the rotation of polarized light by a twisted nematic liquid crystal alignment and the disappearance of this effect when an electric field is applied across the device. Twisted nematic liquid crystal devices typically comprise a suitable liquid crystal composition confined between two optically transmissive plates, the plates having transparent conductive films affixed to their surfaces facing one another in the device. The alignment of the surface layers of the liquid crystal in the "off" state of the device is determined by the interaction of the liquid crystal composition with the confining surfaces of the display device. The orientation of the surface layers of the liquid crystal is propagated throughout the bulk of the composition.
To effect orientation of a confined liquid crystal, the internal surfaces of the conductive plates of a sandwich display device can be prepared by unidirectionally rubbing the surfaces prior to fabrication of the device. The liquid crystal molecules immediately adjacent each rubbed surface tend to orient themselves in the same direction as the rubbing. By arranging the opposing conducting plates with the axis of the rubbed surface at, for example, right angles to each other, the liquid crystal molecules at points intermediate the two plates will orient themselves to a degree which is a function of the distance from the two plates. Accordingly, in this example, the liquid crystal will align itself in a continuous spiral path that twists through the 90.degree. angle between the opposing plates.
If the light-rotating liquid crystal "sandwich" is mounted between two crossed light polarizer elements, polarized light will pass into the device and be rotated through a 90.degree. angle as it is transmitted through the twisted nematic crystal composition from one surface of the device to the other. Due to the 90.degree. light rotation effected by the twist of the liquid crystal, the polarized light will be set to pass through the second crossed polarizer mounted on the opposing side of the display. In the prior art, it is known that by positioning a light reflector behind the second polarizer, the polarized light can be reflected back through the second polarizer to pass through and be rotated by the confined liquid crystal and then exit out the first polarizer where it was introduced.
When an electric field is applied across the liquid crystal composition between the two conductive plates, the twisted orientation of the liquid crystal is obliterated as the molecules align themselves with the applied field. As the liquid crystal is untwisted, polarized light entering the device through the first polarizer will no longer be rotated 90.degree. as it is transmitted through the liquid crystal. Therefore, the non-rotated light will be unable to pass through the second polarizer which is set correspondingly crossed to the first polarizer. Selective application of voltages across discrete segments of the liquid crystal device can readily accomplish patterns of bright areas (no applied electric field, resulting in reflected light) and dark areas (applied electric field, resulting in no reflected light).
Operation of the liquid crystal display device, as described above, is in part dependent upon the optical character and intensity of the light introduced into the device. Under conditions of sufficient ambient light, the reflective illumination arrangement is adequate. However, reduced ambient light may diminish suitable contrast for the display device. Such reduced contrast is addressed in the prior art by supplying an internal, supplemental light means to enhance illumination and make the display more desirably readable. However, the incorporation of supplemental light sources adds undesirable bulk to the display and increases power requirements. With regard to power requirements, it will be readily appreciated that in conventional devices employing LCD displays (e.g., wrist watches, calculators, personal digital assistants, cellular telephone displays, and laptop computers) backlighting and edge-lighting are oftentimes the greatest source of power drain. With the attractive features of "compactness" and portability diminished by addition of supplemental light and bulky power sources, there is need for a display that is adequately viewable under ambient light without requirement of supplemental edge-lighting or backlighting.